Why not use CarbonCopyCloner to do a "clone" of your existing system to a USB flashdrive?
If you want to keep it "bare bones", you can deselect items in CCC that you do not want copied.
Once completed, you will have a fully-bootable version of the OS on the flash drive. It may not be "completely virgin", but it will be usable, and "restorable", if something ever goes wrong with your main drive...